of Edinburgh, Session 1883 - 84 . 509 
the grains of glauconite, which chiefly give the green colour to these 
sands. 
Red Muds . — In some localities, as for instance off the Brazilian 
coast of America, the deposits differ from blue muds by the large 
quantity of ochreous matter brought clown by the rivers and 
deposited along the coast. The ferruginous particles when mixed 
up with the argillaceous matter give the whole deposit a reddish 
colour. These deposits, rich in iron in the state of limonite, do 
not appear to contain any traces of glauconite, and have relatively 
few remains of siliceous organisms. 
Volcanic Muds and Sands . — The muds and sands around vol- 
canic islands are black or grey; when dried they are rarely coherent. 
The mineral particles are generally fragmentary, and consist of 
lapilli of the basic and acid series of modern volcanic rocks, which 
are scoriaceous or compact, vitreous or crystalline, and usually pre- 
sent traces of alteration. The minerals are sometimes isolated, 
sometimes surrounded by their matrix, and consist principally of 
plagioclases, sanidine, amphibole, pyroxene, biotite, olivine, and 
magnetic iron ; the size of the particles diminishes with distance 
from the shore, but the mean diameter is generally 0"5 mm. Glau- 
conite does not appear to be present in these deposits, and quartz 
is also very rare or absent. The fragments of shells and rocks are 
frequently covered with a coating of peroxide of manganese. Shells 
of calcareous organisms are often present in great abundance, and 
render the deposit of a lighter colour. The remains of Diatoms and 
Eadiolarians are usually present. 
Coral Muds . — These muds frequently contain as much as 95 per 
cent, of carbonate of lime, vt^hich consists of fragments of Corals, cal- 
careous Algae, Foraminifera, Serpulse, Molluscs, and remains of other 
lime-secreting organisms. There is a large amount of amorphous 
calcareous matter, which gives the deposit a sticky and chalky char- 
acter. The particles may be of all sizes according to the distance 
from the reefs, the mean diameter being 1 to 2 mm., but occasionally 
there are large blocks of coral and large calcareous concretions ; the 
particles are white and red. Eemains of siliceous organisms seldom 
make up over 2 or 3 per cent, of a typical coral mud. The residue 
consists usually of a small amount of argillaceous matter, with a 
few fragments of felspar and other volcanic minerals ; but ofi‘ 
